1. Field of the Invention
The present invention relates to a centrifugal compressor, and more particularly, to a centrifugal compressor having an improved operation structure to improve durability and bearing-support performance.
2. Background of the Related Art
In general, a centrifugal compressor is an apparatus for compressing and pumping a fluid by suctioning the fluid in an axial direction of a rotating impeller and ejecting the fluid in a circumferential direction thereof.
FIG. 1 is a front view of a conventional centrifugal compressor, and FIG. 2 is a side cross-sectional view of a structure of the conventional centrifugal compressor. As shown in FIGS. 1 and 2, the conventional centrifugal compressor includes a rotary shaft 11, an impeller 20, a shroud 30 and a volute chamber 40.
Here, the impeller 20 is connected to the rotary shaft 11 connected to a motor to be rotated. Accordingly, a fluid is suctioned in an axial direction of the impeller 20 through a suction port 10 to be ejected in a radial direction. In addition, the shroud 30 is disposed to surround the impeller 20, and the ejected fluid is collected in the volute chamber 40 disposed in a circumferential direction of the shroud 20.
Here, the impeller 20 is provided by separately assembling front and rear members.
In addition, one surface 21 of the impeller 20 includes a plurality of blades 20a having a rounded cross-section and configured to be rotated to suction a fluid. As the impeller 20 is rotated in an arrow direction shown in FIG. 1, the fluid in contact with the one surface 21 is accelerated to be centrifugally compressed and ejected in a radial direction. The fluid accelerated as described above is guided by the shroud 30 to be radially ejected, and the ejected fluid is collected in the volute chamber 40 having a ring shape and disposed at a circumferential end of the shroud 30.
The fluid collected in the volute chamber 40 is moved along the volute chamber 40 with inertia in a rotating direction of the impeller 20 and then ejected through an ejection port 45. Here, a cross-section of the volute chamber 40 is configured to increase in the rotating direction of the moving fluid.
As described above, as the impeller 20 is rotated to suction the fluid through the suction port 10, and press and eject the fluid through the ejection port 15 using a centrifugal force, continuously performing compression and pumping operations of the fluid through the centrifugal compressor.
Meanwhile, since the fluid at the one surface 21 of the impeller 20 is accelerated by the centrifugal force to lower a pressure, the pressure at the one surface 21 of the impeller 20 is lower than that at the other surface 22 opposite to the one surface 21. As described above, when the pressure at the one surface 21 of the impeller is lower than that at the other surface 22, an axial thrust force is applied to the other surface 22 of the impeller 20 in the arrow direction by the pressure difference. In addition, the fluid having a pressure increased through a gap between the impeller 20 and the shroud 30 is introduced in the arrow direction shown in FIGS. 1 and 2, and thus, the axial thrust force is further increased.
In order to solve the problem, as shown in FIG. 3, a conventional double suction centrifugal compressor includes impellers 20 disposed at both ends thereof. As the impellers 20 are rotated, a fluid is suctioned through suction ports 10, and compressed and ejected through ejection ports 45 by a centrifugal force, respectively. Here, in the double suction centrifugal compressor, axial thrust forces applied from the impellers 20 disposed at both ends are offset from each other.
Here, a foil-type gas bearing includes a bump foil 3 and a top foil 4 overlapping to surround the rotary shaft 11 and a thrust bearing disk 50. When the rotary shaft 11 is rotated, a dynamic pressure due to an air flow is formed in a space between the foils and the rotary shaft 11. By the dynamic pressure of the air, the foils are resiliently deformed in a direction away from the rotary shaft 11, and an air gap is formed between the rotary shaft 11 and the foils so that the rotary shaft 11 can be rotated without friction with the foils.
However, the conventional centrifugal compressor has the following problems.
First, while the axial thrust forces may be offset when the two impellers are used to offset the axial thrust forces, a resistance due to a parallel operation occurs, which decreases performance thereof.
Second, due to the axial thrust forces, the impeller may impact the shroud and cause friction to decrease durability, and vibrations caused by the impact and friction may cause noises. Here, since the axial thrust forces are further increased as the rotational speed of the impeller is increased, a support load of a thrust bearing 13 enduring the increased axial thrust forces and supporting the rotary shaft is increased, and thus, a dynamic support structure must be reinforced.
Third, a disk 50 installed in the thrust bearing 13 may cause heat generation, wearing and power loss caused by breakage of the air gap and an increase in temperature due to partial contact and friction at a concave and convex part of the top foil 5 of the thrust bearing 13 according to rotation of the rotary shaft. Since such an abrupt increase in temperature eventually decreases performance of the thrust bearing 13 so that the axial thrust forces generated at the impellers cannot be controlled, a blow off valve (BOV) 80 for removing a surging phenomenon must be provided.
Fourth, a high temperature air compressed by the impellers is partially transmitted to the volute chambers 40 as shown by arrows, and the remaining gas is transmitted to rear sides of the impellers 20 through gaps between the impellers 20 and the volute chambers 40, and then sequentially transmitted into the thrust bearings 13 and radial bearings 60 to accelerate an increase in temperature of the gas bearing, decreasing durability of the centrifugal compressor.